The background description provided herein is for the purpose of generally presenting the context of the disclosure. Work of the presently named inventors, to the extent it is described in this background section, as well as aspects of the description that may not otherwise qualify as prior art at the time of filing, are neither expressly nor impliedly admitted as prior art against the present disclosure.
Holographic storage systems provide high-capacity non-volatile data storage. In holographic storage systems, a source of monochromatic coherent radiation is split into an object beam and a reference beam. The object beam is spatially modulated by a data source that provides data to be stored on a holographic storage medium (medium). The reference beam is unmodulated. During write operations, the object and reference beams are directed onto a region of the medium. The object and reference beams interact to generate an interference pattern called a hologram. The medium is made of a light sensitive material that records the hologram.
During read operations, the medium is illuminated by a read beam having characteristics of the reference beam. An image sensor converts light signals received from the medium into read back signals. An image of the recorded hologram can be constructed from the read back signals.
Referring now to FIG. 1, a functional block diagram of a holographic storage system is 10 is shown. The holographic storage system 10 comprises a medium 12, a drive apparatus 14, a laser unit 15, an optical pickup unit (e.g., an image sensor) 16, a write module 18, a read module 20, a control module 22, and a host interface 24.
The control module 22 controls the drive apparatus 14. The drive apparatus 14 drives the medium 12. The drive apparatus 14 controls positioning of the laser unit 15 and the image sensor 16 relative to the medium 12 during read/write operations. The laser unit 15 generates laser beams (e.g., the object, reference, and read beams) for writing and reading data on the medium 12. The image sensor 16 senses images stored on the medium 12 and generates read back signals.
The write module 18 includes encoders (not shown) that encode data and modulators (not shown) that modulate the encoded data. The write module 18 generates write signals that are input to the laser unit 15. The laser unit 15 writes data on the medium 12 based on the write signals. The read module 20 receives read back signals from the image sensor 16. The read module 20 includes demodulators (not shown) that demodulate the data and decoders (not shown) that decode the demodulated data.
The control module 22 communicates with a host (not shown) via the host interface 24. The control module 22 receives data from the host that is to be written on the medium 12 and inputs the data to the write module 18. When the host requests data from the medium 12, the control module 22 returns the data read by the read module 20 from the medium 12 to the host.
Over time, the medium 12 may deteriorate due to repeated read/write operations and develop defects. Additionally, the medium 12 may develop defects due to scratches, dust, etc. The defects are called media defects. Media defects can adversely affect characteristics (e.g., amplitude) of the read back signals. Consequently, the data read back from the medium 12 may not accurately represent the data written on the medium 12.